1. Field of the Invention
This invention relates to the field of radio frequency and microwave system design, and more particularly to a method of defining radio frequency and microwave system component lengths.
2. Description of Related Art
The efficiency of radio frequency or microwave systems which utilize waveguide structures to carry information is limited by the inherent tendency of electromagnetic radiation to reflect at impedance discontinuities in such systems.
Each interconnection point presents discontinuities to the power flowing through the path. The discontinuities cause some of the power to be reflected and travel the path in a reverse direction, which results in a standing wave pattern of energy in each structure. On the system level, much of the distributed reflected energy will show up as energy reflected from the system input, while a fraction of the energy will be converted into heat losses. Both the reflected energy and the heat losses reduce the amount of power available at the system output, and therefore the efficiency of the system.
This reflective phenomenon may be quantized in a variety of ways, e.g., voltage standing wave ratio (VSWR), return loss (RL), or insertion loss (IL), each of which is an expression of lost or reflected power. However, while each radio frequency or microwave system component and interconnecting waveguide or cable has an intrinsic VSWR, RL, and IL, the response characteristics of a system made up of such components is determined not only by the response characteristics of the individual components, but also by the manner in which the individual components are interconnected.
Power loss specifications contain no phase information, and thus cannot be used to predict the manner in which voltage phasors of the reflected waves will combine and propagate through the system, increasing the overall system loss. The standing waves which give rise to VSWR losses interact with each other on a system level, causing in-phase voltage phasors of each component to add, creating a much higher system VSWR.
This interaction of voltage standing wave ratios can be viewed as a situation where two half-silvered mirrors are in the power path facing each other. The power that is transmitted through the pair is reduced or enhanced due to the combination of the voltage vector reflections bouncing infinitely many times between the reflective faces.
It will be appreciated that such interaction is clearly dependent on the relative phasing of the reflection vectors at each mirror's face. Since voltage standing wave ratio is a magnitude relationship and does not contain phase information, the exact impact of this interaction cannot be predicted from VSWR specifications or measurements. The worst case situation can, however, be calculated from voltage standing wave ratios as a function of a maximum in-phase situation, thus enabling accurate prediction of the worst case uncertainty in measured power values.
The most common method for minimizing power lost through reflection and conversion into heat is through impedance matching techniques. When the impedance of one section on the circuit is matched to the impedance of the previous section, usually by using the complex conjugate of the impedance of the previous section, maximum power transfer is achieved by reducing the degree of discontinuity between devices at which reflection occurs.
Impedance matching techniques are complicated, requiring both magnitude and phase information for individual components, and are limited in the frequency range across which they can perform their desired function. Because impedance is a function of frequency, the broader the frequency band of operation for the system, the harder the impedance matching technique is to implement.
Impedance matching techniques are especially impractical in systems, such as test systems, which are designed to respond to the entire coaxial operating frequency range, presently considered to be DC to 40 GHz in reasonably mature technology, and in which most component specifications, with the exception of cable lengths, are dictated by requirements relating to the function of the system and therefore are not variable by the system designer for the purpose of minimizing reflections. These types of systems employ architectures that are low-loss, bidirectional, and have little reverse isolation from one section of the circuit to the next.
The extremely broad frequency response required of such systems precludes the use of impedance matching techniques to minimize reflections. Other conventional methods, for example, those which depend on selecting appropriate component VSWR or RL specifications in certain limited frequency ranges, are also inadequate to control the interactions of the distributed reflections in extended bandwidth systems, and thereby minimize system level reflections across the entire frequency range.
Even when impedances are matched to the greatest extent possible, the use of identical components, or cables of identical lengths to interconnect the components, presents the situation of a "periodic" structure. At certain frequencies, the reflection from the far end of, for example, a cable or a cable-switch structure, will combine in phase with the reflection from the near end of the structure, and this composite reflection will propagate through the system circuit in the reverse direction. Such in-phase combinations create a situation of high system reflection and high system losses due to the distributed circuit reflections, causing sharp power losses at the frequency or frequencies at which the reflections combine in phase.
Furthermore, when cables having different, but wavelength-related lengths are used, the same situation can arise. For example, assume that an 8 inch cable and a 14 inch cable are used in the system path. It will be noted that 14 is not an even multiple of 8. Nevertheless, for wavelengths of 4 inches, 2 inches, 1 inch, 1/2 inch, and so forth, the power reflected from the far end of the cable will combine in-phase with that from the near end of the cable, for both cables. For wavelengths close to the nominal values, nearly in-phase combinations will occur.
This is referred to as a "periodic situation due to multiplicity" and results when one cable or component length is a multiple or a combination of other cable or component lengths. In order to prevent this situation from arising, all possible periodic or multiplicity relationships between elements of the system must be eliminated. Because of the large number of different possible relationships between numbers which might give rise to multiplicity situations over the frequency range of the system, however, it has heretofore been virtually impossible to design a system which completely eliminates such relationships over a wide range of frequencies.
The broadband, low loss nature of the test systems referred to above present special difficulties in finding a solution to this problem. The consequence of the limitation of working only with cable lengths is that a total and complete solution cannot possibly be realized because repetitions of the same switch or other component in a path will always present a degree of periodicity to the power flowing in the path. Nevertheless, weakening the periodic structure by altering the cable lengths used to construct a path would be highly advantageous.